Varistor curve tracer



Feb. 16, 1954 R. R. BLAIR 2,669,004

VARISTOR CURVE TRACER Filed Nov. 21, 1952 EXTERNAL OSCILLOSCOPE A 7'7'ORNE V Patented Feb. 16, 1954 TED PAT E N T OF P ICE VARISTOR- CURVE TRAGER Rcycrii: Blair; Berkeley Heights, N. J., assignor to Bell. Telephone. Laboratories, Incorporated, "New-York; N.*Y.',' a cornoration of New York .iApplication'November 21, 1952, Serial No. 321,796

(Cl. 292 5.3Y

'7" claims.

v iT-hisinvention relates-to-apparatus for displaying the complete current-voltag characteristic of a varistor onacathode-ray oscilloscopeand for applying electrical .iorming pulses .to the varistor. I A varistor .may =be definedasa twopole .-semiconductor, such as apiece. of germanium, silicon, etc., which ma bcmixd. with. a small proportion ot-some other. substance.

, .Qne object ofthe invention. isapparatus-v for displaying both -the .forward, and the. reverse, current-voltage characteristics of a varistor on the screen. of a-cathode-ray oscilloscopein such rapid alternations-that, .due to the. persistency of vision, the two characteristics combine to form one complete characteristic;

Anotherobject isapparatusforapplyingto a varistor electrical pulses. forforming the v'aristor and. for displaying a. characteristic of. the varistor during the forming process.

feature-of the invention is thesprovision.

of'means for changing the voltage andcurrent scales of the characteristics, sothat both characteristics maybe displayed .atone. time on the mostadvantageousscales.

Another feature-of the invention-is the .provision of means for electronically timing the time intervalvof the applicationof theelectrical formingpulsesl I A further feature oi. the invention is. the provisionof means for; detecting the sudden drop inethe reverse resistance ofthevaristor, when the: forming pulse .have warmed the varistor sufiiciently to reduce thelpeak reverse voltage below the forming voltage.

The drawing discloses-a typical ..embdiment otthe invention. W M

The ci-rcuit is switched for successive and alternate displays of the two characteristics by the relays S l 2, SI 3,: which: may conveniently. .be knowntypesoi mercury contact. relays. 'Since the characteristics -are displayed to smal1 ..fractions'of a-vo1t;-the switching. device should. have a; very lowpassingqresistance. and. no contact difierence of'potential, thus, mercury contact relays are "preferred overother switching .de-

vices; such as electronic switches,- var'istor gates, etc. The winding of relay S12- is energized froin the secondary winding of transformer through adjustable resistors 3,4. Resistors 3, 4, with capacitors 2, 5, forfn a phase shifting bridge circuit,.which maybe adjusted to produce operatioh cf the relay. 'at a desired point, preferably zero, orithc applied wave. The primary winding of transformer TRfigis connecte tea-source of commercial emanating-power. The secondary winding -cftransformer 'I'Rt time-any com f2 nected to .asimilar phase. shifting bridgetclr'ciiit associated by the'fifthand sixth sections of switch sl5, with the windingofrelay' Sl3.

Inamercury. relays SIZ, S l3,'.-ther'e is av small stray capacitance between the relay coilsyand the relay contacts, which might result inpithe transmission of an alternating voltage to the relay contacts which would interfere with the accuracy of the test. results. To avoidthis result, the circuit supplying the windings of relays SIZ, Sl3,.is connected toground through the brush of potentiometer P4.

The alternating power supply is connected through an adjustable aut'otransformer I, which may be of thetype .known'commercially as a Variac, .to the primary winding of transformer 'IRl. One secondary winding of transformer 'IRl i connected to the left-contact cfswitch S2; the right contact of switch 'SZ'being connected to theblade of. switch S3. The other sec-' ondary winding of transformer'TRl i connected to the right'contact of switch ,S2,"while a'tap from this winding is connected to'the left contact ofswitch. S3. With-switch S2011" the left contact, a low voltage is selected; with switch S2 on the right contact and switch1S3 on the left contact, a medium voltage is selected; with switch S2 on the left contact and sw'itch sii' on the. right contact, a high voltage is selected. Thus, by manipulationof switches s2,"S3,- and the "Variac, a wide range of voltages may be selected. 'The resistor R3 and gas-filled glow lamp V2 connected in serial relationship across the output circuit indicates when voltages in excess of 50 volts are developed acrossthe'varistor, to

' warn the operator of the apparatus to exercise caution.

In the operated po'sition 'of re1aysSl2f-S13, current flows from ground through'the selected secondary winding of transformer TRI, switch $2, the first section of switch =Sl5fthe resistor selected by switch S4, the second and third-sections of switch Sli'i, make contacts of relays", fourth section of switch Sl5,'fourth contact and blade of switch Sit-D, in the blocking direction through the varistor under test, blade and fourth contact of switch Sill-C, resistor Selected *by switch S5, right make contact and swinger of relay S13 to ground. The vertical'platesbf the cathode-ray oscilloscope areconnected through the fourth contact and blade of-switch"Sl0- B and terminafv across the resistor selected by switch S5, and are thuszenergized by a voltage proportional to-Ithe :currentrfiowing:Tthrough the Var-181301.

'if fhecathode-raysoscilloscopeemay baanysuin 3 able device, and should include the direct-current amplifiers, controls, and power supply usu ally furnished with such devices.

In a practical embodiment of the invention, with a Widely used commercial oscilloscope, the resistors associated with switch S5 had graduated values from less than one ohrn to over fifty thousand ohms.

Alternating currents also flow through resistor R22 and the resistor R23 to R21, associated with switch S9, through the left make contact of relay S13 to ground. The horizontal plates of the oscilloscope are connected through the fourth con tact of switch Sl-F and terminal I-I across the resistor selected by switch S9. As rather high voltages may be required to force a suitable current through the varistor in the blocking direction, resistor R22 and the resistor selected by switch S9 form a voltage divider to reduce this high voltage to a value suitable for application to the horizontal plates. Resistor R22 may have a resistance of 100,000 ohms, and the resistors associated with switch S9 may have graduated values from a few hundred ohms to several ten thousands of ohms.

The current from the grounded secondary winding of TRI flows through the resistor selected by switch S4, the varistor under test, and the resistor selected by switch S5 to ground. The series resistors selected by the switch S4 which have graduated values from a few hundred ohms to a megohm are inserted in the circuit as a safety precaution to limit to a safe value the current that can flow when the applied voltage carries the varistor characteristic into the region where the resistance of the varistor falls to a low value. The vertical plates of the oscilloscope are connected across the resistor selected by the switch S5 and are responsive to the current flowing through the varistor. Resistor R22 and the resistor selected by the switch S9 are connected as a potential divider across both the varistor under test and the resistor selected by switch S5; and the horizontal plates of the oscilloscope are connected to this potential divider. The horizontal plates thus are responsive to the voltage drop across both the varistor and the resistor selected by switch S5; but, as the resistance of the resistor selected by switch S5 is much less than the resistance of the varistor, the error is slight. In prior units, the circuit was grounded at the junction of the varistor and the resistor; but, this connection, in effect, connected a portion of the distributed capacity of the transformer winding across the varistor resulting in phase shift which caused the trace on the oscilloscope to open out into a loop. The present connection eliminates this undesirable effect.

The secondary winding of transformer TR3, and one secondary winding of transformer TRA are connected, in series aiding, through the winding of potentiometer P1. Resistor R2 and pilot lamp L2 are connected in series across the secondary winding of transformer TR3 to indicate that power is being supplied to the equipment. 1

-In the unoperated positions of relays S12, S13, current will flow from the brush of potentiometer P1, through both break contacts of relay S13, switch S6 and the resistor associated there with, fourth contact and blade of switch SIB-C, through the varistor under test in the passing direction, blade and fourth contact of switch S'1i!D, fourth section of switch S15, break contact of relay S12, back to the winding of transformer TBA; The vertical plates of the oscillo- '4 scope are connected through the fourth contact and blade of switch Slil-B, and the break contacts of relay S13 across the resistor selected by switch S6. Current will also flow from the brush of potentiometer P1, right break contact and swinger of relay S13, through ground and the horizontal plates of the oscilloscope, resistor R22, fourth section of switch 15 and break contacts of relay S12 to the winding of potentiometer P1, to produce the horizontal sweep of the oscilloscope. As the voltages applied to the varistor in the passing direction are usually fairly low, it is not essential to use a voltage divider ahead of the CRO for this supply, as the potentiometer P1 may be adjusted to supply a suitable voltage.

By adjustment of switches S4, S5, S6, S9, the most suitable scales may be chosen for the display of the characteristics on the screen of the oscilloscope, and different scales may be used for the forward, and the reverse characteristics. Relays S12, S13, are operated at the frequency of the power supply, thus, due to persistency of vision, the two characteristics appear simultaneously as one complete characteristic. The phase shifting networks may be adjusted to operate relays S12, S13, at the proper point on the alternating wave.

The vertical and horizontal input impedances of the cathode-ray oscilloscope should have proper values for use with the resistors associated with switches S5, S5, S9. With the vertical sensitivity of the oscilloscope adjusted to 0.05 volt per inch of deflection on the screen, the resistors associated with S5 and S6 are selected to produce current ranges of 1, 10, microamperes and 1, 10, 100, 1000 milliamperes. With the horizontal sensitivity of the oscilloscope adjusted to 0.83 volt per inch of deflection, the scale for the forward voltage is one volt per inch; and the resistors associated with switch S9 may be selected to provide scales of l, 5, 10, 50, 100 and 500 volts per inch for the reverse voltage.

Some semiconductor diodes have very small reverse currents, and, when this condition is associated with appreciable shunt capacitance, either in the diode itself, or in the wiring, the reverse trace on the oscilloscope tends to open into a loop due to phase shift. Entrapped moisture in point contact diodes may also produce a similar effect. To eliminate this effect, the varistor characteristic may be traced at slow speed, much slower than the normal tracing at sixty cycles. In such cases, to avoid heating of the diode due to forward current, the display of the forward characteristic is suspended.

The tapped secondary winding of transformer TR4 is connected to the anodes of the rectifying diodes V3; the other secondary winding being connected to the heaters. The cathodes of the diodes V3 are connected to the filter formed of resistors R32, R31, and capacitor C1, the voltage regulator V4, and the filter formed of resistor R35 and capacitor C2. Switch S15 is moved to the left-hand contacts, completing a circuit through resistor R45, left contact of fifth section of switch S15, winding of relay S13,left contact of sixth section of switch S15, locking up relay S13 in the operated condition.

Alternating current from the selected secondary winding of transformer TRl flows through switches S2, S3, and is rectified by the rectifier VR2. The rectified current flows through the left contacts of the fourth section of switch S15, makes-contacts of relay S12, .left' contacts of third section of switch'Sl5, filter formed of re .5 '3l8,-2capac1;tornC i; sinductoraimr-oapacitor resistor R45; def-t contacts do second section mfzswitchisflfi, resistor selected by'J-switch S4, left contacts of first sectionrofaswitchfiliidfonrtlroontactsi-ioi switchssmal),sthroughithe'ivaristor-zunder etcstxi ithenreverse .idirection; fourth contacts bi szwitch S|ifl=-'C, resistor selected ..-by iswitohssfi, breakzcontaictsof relayfiltgback to the. secondary mindingtfof dransformeriTBl 1 By using. a, capacdmtiive; input. toffl'she .filter, the amplitude of the ifitteredi voltage-may; be "made requal to thei-peak evaluator etheualternating avoltage .used Kineiter- -natingscurrent :tests,"athus giving the :same peak ideflection .on the oscilloscope :for. .ieither :position ofdswitclrsi'l 5.

fcilihef *fOUIthL-ICOIEQJCtsrOf SWiWhQSlfl-EI are conmected' =:through ithe zrfourth contacts of switch and terminarV tothe werticalplates-cf the oscilloscope; and the fourthtcontacts of switchss l e-FD rare.'connectedithrough' resistor R2 2, dourthbontacts of switch $113 and terminal H; FOTHl'JhEhOIiZOHfiaI plates of the. oscilloscope. =By imanipulation of-the yaria blevtransformer 'l, the filteredourrentiflowing through the varistor may be gvaried over the --range .of the characteristic of 'ftheazaristor.

:fiiherbrushotpotentiometer Pi is i'left' un- .agrounded .rby the :sbreak :contacts of relay SI 3, thus the operation of relay s12 does not supply any:alternatingncurrent in the forward direction comhe varistor.

zf'iiheiswitch'rts'lil thasFfive positions, first Zero, second .Cal. :V,ithird, CaL: H, fourth View, fifth the fir'staposition, switch S l fl-B grounds the; terminal, and switch 6 ne r grounds the H i -ter1i1inal,-.-so that the oscilloscope-may be adjusted di'oi-ZEEO. r-In' thesecond position, switch'Sli3-B applies a standard voltage from the brush of potentiomet'er 1 6 to the-V terminal, while switch continues to ground the H terminal, so

that .the sensitivity oi the oscilloscope in the verticalidi-rectionxmay be calibrated. In the third @position,.:switch:S-'iu-B grounds the V terminal, whilerswitch Siu-F applies a standard voltage dromthebrushof potentiometer?! to the H terminahso that the sensitivity of the oscilloscope in the horizontal direction may be calibrated. As described herein above, in the iourth'po'sitionof switch 28w, the varistor characteristic'may be viewed. vThe switch 558 selects one of four --forming. processes-and is moved to the desired position before switch SID is moved to the fifth pos'ition.

The transformer may conveniently be a known type of adjustable voltage device. switch S8 in the first position, alternating currents =will' fiow "from ground through TR5,-'-first contact and blade oi-switch S8--2,"fi ft'h contact and blade of switch S lt-D, through the varistor being formed, blade'ansll fifth contact of switch SI lJ-C, and-one-of the resistors-selected by switch S'hto; ground. --The resistors selected by switch S1 may be graduated in resistance from say five ohms to one hundred ohms, so that by adjustment of transformer IE5 and switchSl, ade'sired value of current wi'lrfiow in the'varistor. A potential dividen-formed by resistors-5R28, R29, is 001!- nectediaoross the resistorselected by switch S1, and the junction of these resistors is connected through the fifth contact and blade of switch SID-B to the vertical plates of the oscilloscope. At the same time an alternating voltage from one of the secondaries on TRI is applied to the horizontal plate through SIS (right-hand position), a resistor selected by S4, make contact of With iii

Ill)

mnirim-i tcrcbta'in aznorizcntal sweep. tit-nus a c'e is bbtainedwom the oscilloscope which plots var torn-current vs. the voltage sobtained dromtiIRl 'Whilei not' a' ftrue trace, it'isus'eful roe observing varis'tcr current during'aforming.

Withti switch 1-S8 ion ithe second-contact, 'the iforming: process :may hm-automatically timed.

alternating :current when new from mansformer :TRZS: ithrough :th breahcontacts of relay ifs-Ii:andrthersecnndaccntacti of switch S842.

Tubes V5, V6, are gas-filled tetrcde's, of lmown atypqvihavingtthe second-grid connected to the cathode, 311dith8 -cathodesifenergizd "by "the usual supply .:'(not::.shown ."The first grid of .ftube is connecte fgroun'd through. resistor FEM;*iiher' anodeimay:be connected tothe anode supply throughrresistor11?;39, and the fifth" contact and blade of switch SIG-A; and the second grid raml cathode :are connected "to the potential flivi'derrformed of:snesistor R31, R40, connected :across zthe'annodei-supply. 'The cathode of tube iisroonnectedzthrough adjustable: resistor P3 to the first grid of tube W3. trThe anode of tube V6 may be connected to theanode'supply through the winding of relarSdfi, resistorRSfi, and the fifth'contact and :blade of switch S1 fi-A; the secand grid andbathode are connected to the potential'dividerformed rat-resistor Rat and adjustablerresistorPt-connected across the anode supp fm arid are connected through-capacitor "C4 to round.

'When switch' st :is on the second contact, and sewitcli's lfi ismoved to the fifth position positive voltage from thean'cdesupplyis applied through ,thewblade and fifth contact of switch SID-A, re :sistor-Rflhssecond-contact and blade of switch Sit-ii; rectifier 'VRI, to the first grid of tube V5, firing the tube,".andrcausing the potential of the cathodeto rise. :Capacitor C3 is connected from ground through the blade and second contact of switch 5884 to the junction of resistors P3, R3 5, and thus tothe firstgrid of tube V6. Current will. then iiow from the cathode of tube V5 through resistorPS to charge capacitor C3, and, eventually, will raise the potential of the first grid of tube V6 tothe critica'l value firing tube V5, and operating relay SM to out offthe forming current Thecircuit is restored by opera-ting switch sic-to remove'the anode supply from tubes V 5, V6,--and to discharge capacitor C3 throug l-i resistor"R33 and switch S I 0 E.

'ResistorPt may have a maximum resistance of, sayone megohm, andcapacitor C3 a capacitan-cc of :several m'icrofarads, thus the forming period' may be from a-sma'lltraction of a second to 'several seconds.

As a varistor warms up during the forming process, the peak ba'ckvoltage tends to decrease, and, thevp-eakbacky'oltage becomes less than the forming voltage, the varistor is then 010 .eratingin a negative resistance region in which the current rapidly increa-ses, causing an abrupt increase 'in 'lthe internal temperature of the varistor, and a sudden drop in the originally ihi gh.reverse 'resistance to a low value of the order of i the forward resistance. The present apparatus-provides means for producing and observing: this change in" the characteristic of the varistor.

When switch S8 is on the third contact, and switch SI I! is moved to the fifth position current is supplied to the varis-tor, as before, through the contacts of relay SM and switch 88-2. The normal current through the varistor in the reverse, or blocking, direction, is small, thus the positive 7. voltage drop across the resistor selected'by switch S1 is also small. When the reverse resistance suddenly snaps to a low value, the positive volt-- age drop across the resistor selected by switch S1 suddenly rises. This positive voltage is applied through the third contact and blade of switch SB-l, and rectifier VRI, to the first grid of tube V5, which fires, applying a positive voltage to the first grid of tube V6, which also fires, operating relay S l 4 to cut off the forming voltage from the varistor.

If desired, switch S8 may be moved to the fourth contact, connecting capacitor C3, through resistor R34, to the first grid of tube V6. Under these conditions, the forming voltage is supplied to the varistor for a measured time interval after the reverse resistor has snapped to a low value.

A current characteristic of the varistor is displayed on the screen, as described above, during all of the forming processes, and switch SIO is usually restored to the fourth position after any of the forming processes.

What is claimed is:

1. Apparatus for alternately displaying on the screen of a cathode-ray oscilloscope the forward and reverse voltage-current characteristics of a two-pole asymmetrically conductive device comprising first and second conductive elements contacting said device, first and second series resistors connected to said second element and to one of the vertical plates of said oscilloscope, a third resistor connected from said first element to one of the horizontal plates of said oscillo scope, an adjustable resistor connected in series with said third resistor, sources of positive and negative voltage, and switching means for alternately and recurrently connecting the source of said positive voltage from said first element to said first series resistor, said adjustable resistor and the other plates of said oscilloscope, and the source of said negative voltage from said first element to said series resistor and the other plates of said oscilloscope, said series resistors and said adjustable resistor being selected to produce advantageous scale factors for both characteristics of said device.

2. The combination in claim 1 with a fifth resistor, a source of alternating voltage, second switching means for disconnecting said first and second resistors from said second element and for connecting said fifth resistor to said element, and third switching means for disconnecting the sources of said positive and negative voltages from said first element and for connecting said source of alternating voltage to said first ele ment and said fifth resistor.

3. The combination in claim 1 with a fifth resistor, a source of alternating voltage, second switching means for disconnecting said first and second resistors from said second element and for connecting said fifth resistor to said element, third switching means for disconnecting the sources of said positive and negative voltages from said first element and for connecting said source of alternatingvoltage to saidfirst element and said fifth resistor, a relay havinga pair of break contacts connected in series with the connection from said source of alternating cur- 8 rent and a'winding,':and thermionic timing means connected to be activated by said third switching means to energize said relay winding after a predetermined time interval.

a. A varistor curve tracer including a cathoderay oscilloscope having vertical and horizontal deflection plates, first and second sources or alternating voltage of opposite polarity, first and second resistors, switching means synchronized with said alternating voltages for recurrently connecting a first combination of the varistor to be tested 'in series with the first resistor across the first source, and alternately connecting a second combination of said varistor in series with the second resistor across the second source and alternately connecting said vertical plates across said resistors, and means for connecting said horizontal plate alternately across said first and second combinations.

5. A varistor curve tracer including a cathoderay oscilloscope having vertical and horizontal deflection plates, a first grounded resistor connected to the vertical deflection plates and in series with the varistor to be tested, a relay having a pair of break contacts and a winding, a grounded source of alternating voltage connected in series with said contacts across the varistor and first resistor, a second resistor connected in series with the horizontal plates across said source, two gas-filled tubes, each having a cathode, an anode, and a control electrode, a third resistor, a grounded source of direct voltage connected through said third resistor to the anode of the first tube and through said winding to the anode of the second tube, first and second potential dividers connected across said source of direct voltage and respectively to the cathodes of said tubes, an adjustable resistor connected from the cathode of the first tube to the control grid of the second tube, a fourth grounded resistor connected to the signal grid of the first tube, and a rectifier connected from th junction of the varistor and first resistor to the control grid of the first tube, whereby when the heat produced in the varistor produces a sudden diminution in the reverse resistance of the varistor, the tubes are fired to operate the relay and disconnect the source of alternating voltage from the varistor.

6. The combination in claim 5 with a grounded capacitor connected to the control grid of the second tube to delay the firing of the second tube for a predetermined time.

'7. The combination in claim 4 with a source of direct current, a switch connecting said source to said switching means to hold said switching means operated, a variable transformer connected to said first source of alternating Voltage, a rectifier and filter connected from said transformer to th combination of said varistor and said first resistor, whereby slowly varying direct current may be supplied to said varistor.

ROYER R. BLAIR.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,459,849 Stateman Jan. 25, 1949 

